Disc drive memory systems are widely employed in traditional computing environments and more currently in additional environments. These systems are used by computers and devices including digital cameras, digital video recorders, laser printers, photo copiers, jukeboxes, video games and personal music players. Consequently, the demands on disc drive memory systems has intensified. Disc drive memory systems store digital information that is recorded on concentric tracks of a magnetic disc medium. Several discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. Magnets on the hub interact with a stator to cause rotation of the hub relative to the shaft. One type of motor is known as an in-hub or in-spindle motor, which typically has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. The bearings permit rotational movement between the shaft and the hub, while maintaining alignment of the spindle to the shaft. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.
In the past, spindle motors used conventional ball bearings between the hub and the shaft. However, the demand for increased storage capacity and smaller disc drives has led to the read/write head being placed increasingly close to the disc surface. The close proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the disc to strike the read/write head, possibly damaging the disc drive and resulting in loss of data. Further, resistance to mechanical shock and vibration is poor in the case of ball bearings, because of low damping. Because this rotational accuracy cannot be achieved using ball bearings, disc drives currently utilize a spindle motor having fluid dynamic bearings on the shaft and a thrust plate to support a hub and the disc for rotation. One alternative bearing design is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as gas or liquid provides a bearing surface between a stationary member and a rotating member of the disc drive. Wobble or run-out is reduced between the relatively rotating members, and the use of fluid in the interface causes a damping effect. While ball bearings utilize a series of point interfaces, hydrodynamic bearings spread the bearing interface over a large surface area.
Since adjacent gap surfaces of a hydrodynamic bearing are not mechanically separated, a potential for rubbing, wear and impact exists, which can effect bearing performance, reduce bearing life and destroy a bearing. Further, it has become essential in the industry to require disc drives to withstand substantial mechanical shock, one concern being the handling of mobile applications such as laptop computers.
Bearing life is one of the most crucial factors in designing any fluid dynamic bearing. Removal of surface material by wear can change the effective bearing gap, which can change bearing performance. The dynamic performance of a hydrodynamic motor is a function of the bearing gap since gap pressure affects dynamic performance, and hydrodynamic and hydrostatic bearings utilize pressures. Further, during the start-stop cycle of the fluid dynamic bearing motor, one or more adjacent surfaces of the bearing come into contact thus causing adhesive or abrasive wear. The worn metal particles are highly reactive in nature and can act as a catalyst to an oil oxidation process potentially resulting in sludge formation, etc. Also, the worn metal particles themselves can cause surface wear. Moreover, water content may exist in a bearing, resulting in corrosive wear of a bearing surface. Material from a surface of the bearing can break away, which can result in a change in bearing performance, bearing failure, or even be expelled from the region surrounding the motor and damage the disc recording surface.
A key factor that affects bearing life is the selection of the material interface between two adjacent or rubbing surfaces. One method to protect a bearing surface is to apply a hard coating to the surface. An example of such coating is DLC or amorphous carbon, which when comes into contact with other metallic surfaces produces a negligible amount of worn metal, thus allowing a long bearing life. However, the coating process is expensive due in part to handling issues, since a bearing has small parts. Further, coating bearing gap surfaces (i.e. shaft or sleeve surface) may result in a coating thickness variation, a taper, and a variable bearing gap. A bearing gap, in particular sections, must remain uniform and constant. When the bearing gap varies, nonrepetitive runout (NRRO), as well as other bearing performances are effected. Further, there is a trend to decrease the aspect ratio (depth to width ratio) in sleeves to achieve greater recording densities, making precise coating more difficult and expensive. Additionally, gap variations may be specified in design, making a coating process even further difficult. Thus, what is needed is a method or suitable bearing material to preserve bearing surfaces and maintain bearing life.